1. Field of the Invention
The present invention relates to a backlight unit for a liquid crystal display (LCD) device and more particularly to a flexible printed circuit board (PCB) for a light emitting diode (LED) backlight unit and a method of fabricating the same.
2. Discussion of the Related Art
As the society has entered in earnest upon an information age, flat panel display devices, which have excellent capabilities of a thin profile, light weight and low power consumption, and so on, are introduced. For example, the flat panel display devices include an LCD device, a plasma display panel (PDP) device, an electroluminescent display (EL) device and a field emission display (FED) device.
Among these devices, LCD devices are widely used for notebook computers, monitors, TV, and so on instead of a cathode ray tube (CRT), because of their high contrast ratio and characteristics adequate to display moving images. The LCD device is not a self-emission type. Accordingly, the LCD device requires an additional light source. A backlight unit including a light source, for example, an LED, is disposed under a liquid crystal panel of the LCD device such that light from the backlight unit is provided onto the liquid crystal panel to display images.
Generally, the LCD device includes the liquid crystal panel, the backlight unit, a main frame, a bottom frame and a top frame. The liquid crystal panel is a main element for displaying images. The liquid crystal panel includes an array substrate, a color filter substrate and a liquid crystal layer therebetween. A PCB is disposed on edges of the array substrate. The PCB is connected to a gate pad and a data pad on the array substrate via a flexible printed circuit (FPC) or a chip on film (COF).
When the LED is used for the light source of the backlight unit, a rigid PCB, where the LED is disposed, and a flexible PCB for connecting the LED to a driving circuit board are required. The backlight unit including the LED as a light source may be called as an LED backlight unit. An integrated circuit board without the flexible PCB may be used for the LCD backlight unit. However, for considering production costs, two PCB, e.g., the rigid PCB and the flexible PCB, connected to each other are used.
First and second polarizing plates are formed on outer sides of the liquid crystal panel, respectively. The backlight unit is disposed under the liquid crystal panel such that one of the first and second polarizing plates is positioned between the liquid crystal panel and the backlight unit. The backlight unit includes a light source along at least one side of the main frame, a reflective sheet on the bottom frame, a light guide plate on the reflective sheet, and a plurality of optical sheets over the light guide plate. A light source guide member may be further included to guide the light source.
The liquid crystal display panel and the backlight unit are combined using the main frame that can prevent movement of the liquid crystal panel and the backlight unit. The main frame having a panel guide and four corners of the liquid crystal panel is positioned on the panel guides such that movement of the liquid crystal panel is prevented. The top frame cover edges of the liquid crystal panel and sides of the main frame, so the top frame can support and protect of the edges of the liquid crystal panel and sides of the main frame. The bottom frame covers back edges of the main frame, so the bottom frame is combined with the main frame and the top frame for modulation.
The light from the light source of the backlight unit is processed to have a uniform brightness through the light guide plate and the optical sheets and provided onto the liquid crystal panel.
FIG. 1 is a view illustrating an LED backlight unit according to the related art. In FIG. 1, the backlight unit includes a plurality of LEDs 15, a light guide plate 5, a reflective sheet (not shown) and optical sheets (not shown). The LEDs 15 are positioned at a side of the light guide plate 5. The LEDs 15 is attached on a rigid PCB 10. A flexible PCB 20 is attached to the rigid PCB 10. Namely, an end of the flexible PCB is boned onto one side of the rigid PCB 10.
Referring to FIG. 2, the flexible PCB 20 is bent at least once and positioned under the backlight unit (not shown). One end of the flexible PCB 20 is connected to the rigid PCB 10 (of FIG. 1), and the other end of the flexible PCB 20 is connected to an external driving circuit for driving the LEDs 15 (of FIG. 1).
FIG. 3 is a cross-sectional view of a flexible PCB according to the related art, and FIG. 4 is a cross-sectional view of a raw flexible board for a flexible PCB according to the related art.
Referring to FIG. 3, the flexible PCB 20 includes a flexible base film 25, a plurality of copper lines 28, a first gold-plating pattern 32, a first cover layer 35, a copper pattern 38, a second gold-plating pattern 40, a second cover layer 45 and a kapton tape 50. The kaptone tape 50 may be called as a polyimide tape. As mentioned above, one end of the flexible PCB 20 is connected to the rigid PCB, where the LEDs are disposed, and the other end of the flexible PCB 20 is connected to the external driving circuit. The plurality of copper lines 28 are formed on a first surface of the flexible base film 25, and the first gold-plating pattern 32 is formed on the copper lines 28. The first gold-plating pattern 32 is positioned at both ends of the copper lines 28. An electric resistance is reduced due to the first gold-plating pattern 32. In addition, a corrosion problem of the copper lines 28 is prevented due to the first gold-plating pattern 32. The first cover layer 35, which is formed of an organic insulating material or an inorganic insulating material, for protecting the copper lines 28 is formed on the copper lines 28. The first cover layer 35 is positioned at a center of the copper lines 28 such that the first gold-plating pattern 32 is not covered with the first cover layer 35.
The copper pattern 38 is formed on a second surface of the flexible base film 25 which is opposite to the first surface of the flexible base film 25. The copper pattern 38 is positioned at one end of the base film 25. The second gold-plating pattern 40 is formed on the copper pattern 38. The second gold-plating pattern 40 covers the copper pattern 38 for preventing a corrosion problem of the copper pattern 38. The second cover layer 45 including a plurality grooves “hm” is formed on the second surface of the flexible base film 25. The second cover layer 45 covers an entire region of the second surface except a region where the second gold-plating pattern 40. The flexible PCB 20 is easily bent due to the grooves “hm”.
Referring to FIG. 4, a raw flexible board 21 includes the flexible base film 25 and first and second copper layers 27 and 37 on the first and second surfaces of the flexible base film 25. When the raw flexible board 21 is processed to obtain the flexible PCB 20 (of FIG. 3), the second copper layer 37 on the second surface of the flexible base film 25 is removed, so that the copper pattern at one end of the second surface is formed and other regions of the second surface is exposed. Then, the second cover layer 45 (of FIG. 3) is formed on the other regions of the second surface and patterned to form the grooves “hm”.
In the above raw flexible board 21 and the flexible PCB 20, most part of the second copper layer 37 of the raw flexible board 21 is not used for the flexible PCB 20. In addition, the second cover layer 45 is required to obtain the grooves 45. As a result, production costs for the flexible PCB 20 are increased.